1. Field of the Invention
The present invention relates to a signal processing circuit and a signal processing method that use a mixer that generates an intermediate frequency and, more particularly, to a signal processing circuit and a signal processing method that are suitable for use in a transmitting circuit or receiving circuit constituting a communication device such as a wireless access device, and are designed to eliminate distortion in a signal to be processed.
2. Description of Related Art
A transmitting circuit that inputs a local signal output from a local oscillator to a mixer, up-converts a transmission signal to a desired frequency, and transmits the transmission signal is generally used in a transmitter. Since the mixer used to generate an intermediate frequency is a non-linear circuit, IM (Inter Modulation) 3, which is the third order distortion, occurs in an input signal. For this reason, the transmitting circuit generally has a variable amplifier provided at a preceding stage of the mixer and is designed to prevent an input to the mixer from becoming excessive (see, e.g., Japanese Patent Laid Open Publication No. 2000-244353 (Paragraphs 0020 to 0023 and FIG. 1)). More specifically, to prevent occurrence of distortion caused by an excessive input to the mixer, the amplification factor of the variable amplifier is controlled.
The mixer causes distortion not only upon receipt of such an excessive input but also at a low temperature. Accordingly, when the transmitting circuit is used over a wide range of temperatures, distortion in the mixer at a low temperature is a problem. At a low temperature, the characteristics of the mixer degrade, and IM3 degrades a circuit output.
FIG. 1 represents the main portion of an example of a conventional transmitting circuit. A transmitting circuit 500 includes a mixer 502 for inputting and up-converting a transmission signal 501. A local signal 504, an oscillation frequency of a local oscillator 503, is input to the mixer 502. A transmission signal 505 which is obtained after the mixer 502 up-converts the transmission signal 501 is attenuated at a variable attenuator 506. A transmission signal 507 obtained after the attenuation is amplified by an amplifier 508 and then input to a directional coupler 509.
The directional coupler 509 sends a transmission signal 511 which has been branched off from the input signal to a detector 512. A detection output 513 from the detector 512 is input to a low-pass filter 514, and an output 515 from the low-pass filter 514 is input to an operational amplifier 517 through a first resistor 516. An output 518 from the operational amplifier 517 controls the variable attenuator 506.
The operational amplifier 517 has a plus (+) input terminal connected to ground and a minus (−) input terminal connected to one end of the first resistor 516. A second resistor 519 for feedback which determines the gain is connected between the minus input terminal and an output terminal of the operational amplifier 517.
In order to keep an original transmission output 521 from the directional coupler 509 around a fixed value regardless of temperature, this conventional transmitting circuit 500 causes the transmission signal 511 to branch off, detects the signal level with the detector 512, and feeds back, to the variable attenuator 506, the detection result as the output 518 from the operational amplifier 517. If the transmission output 521 from the transmitting circuit 500 decreases, the output 518 changes to reduce the attenuation of the variable attenuator 506. On the other hand, if the transmission output 521 from the transmitting circuit 500 increases, the output 518 changes to increase the attenuation of the variable attenuator 506.
However, in the conventional transmitting circuit 500 shown in FIG. 1, when the environmental temperature becomes low, distortion in the mixer 502 increases, and the quality of the transmission output 521 degrades. To cope with this, there has been devised a method of equally suppressing the signal level of the transmission signal 501 regardless of temperature. However, since the signal level of the local signal 504 outputted from the local oscillator 503 is constant, this method relatively increases the proportion of local leakage to degrade the transmission output 521. If the gain of the amplifier 508 decreases at a high temperature, the gain of the transmission output 521 becomes insufficient while the signal level of the transmission signal 501 is kept low.
The problem of temperature-induced distortion in a transmitting circuit has been introduced. A receiving circuit that uses a mixer suffers similar problems.